The present invention relates to a combustion-type power tool, and more particularly, to a combustion-type fastener driving tool in which liquidized gas is ejected from a gas canister into a combustion chamber, mixed with air and ignited to drive a piston, thus generating power to drive nails or the like.
A conventional combustion-type driving tool generally includes a housing, a handle, a trigger switch, a head cap, a combustion-chamber frame, a push lever, a cylinder, a piston, a driver blade, a motor, a fan, a gas canister, an ignition plug, an exhaust-gas check valve, an exhaust cover, a magazine, and a tail cover. The head cap closes one end of the housing and is formed with a combustible gas passage. The handle is fixed to the housing and is provided with the trigger switch. The combustion-chamber frame is movable in the housing in the lengthwise direction thereof. The combustion-chamber frame is urged in a direction away from the head cap by a spring, and one end of the combustion-chamber frame is abuttble on the head cap against the biasing force of the spring.
The push lever is movably provided at the other end of the housing and is coupled to the combustion-chamber frame. The cylinder is secured to the housing and in communication with the combustion-chamber frame. The cylinder guides the movement of the combustion-chamber frame and is formed with an exhaust port. The piston is reciprocally movable in the cylinder. While the combustion-chamber frame has its one end abutting on the head cap, the piston defines a combustion chamber in cooperation with the head cap, the combustion-chamber frame and the end portion of the cylinder, the end portion being positioned near the head cap. The driver blade extends from the end of the piston which faces away from the combustion chamber toward the other end of the housing.
The motor is supported on the head cap. The fan is fastened to the motor and provided in the combustion chamber. The fan mixes the combustible gas with air in the combustion chamber for promoting combustion. The fan also serves to introduce an external air into the housing when the combustion-chamber frame is moved away from the head cap for scavenging within the combustion-chamber frame, and at the same time serves to cool an outer peripheral side of the cylinder. The gas canister is assembleable in the housing and contains liquidized combustible gas that is to be ejected into the combustion chamber through a combustible gas passage formed in the head cap. The ignition plug is faced to the combustion chamber to ignite a mixture of combustible gas and air. The exhaust-gas check valve selectively closes the exhaust port. The exhaust cover covers the exhaust gas check valve for directing the exhaust gas in the axial direction of the tool.
The magazine is positioned at the other end of the housing and contains fastening elements such as nails. The tail cover is interposed between the magazine and the push lever to supply the fastener from the magazine to a position of a moving locus of the driver bit.
In order to provide a hermetic state of the combustion chamber when the combustion chamber frame is brought into abutment with the head cap, a seal member (seal ring) is provided at a predetermined position of the head cap for intimate contact with an upper portion of the combustion-chamber frame and another seal member (seal ring) is provided at the cylinder near the head cap for intimate contact with a lower portion of the combustion chamber frame.
Upon ON operation of the trigger switch while the push lever is pushed against a workpiece, combustible gas is ejected into the combustion chamber from the gas canister assembled in the housing. In the combustion chamber, the combustible gas and air are stirred and mixed together by the fan. The ignition plug ignites the resultant mixture gas. The mixture gas explodes to drive piston for driving the driver blade, which in turn drives nails into a workpiece such as a wood block. After explosion, the combustion chamber frame is maintained in its abutting position to the head cap for a predetermined period of time. During this abutting period, the exhaust gas check valve is closed after the combustion gas is exhausted to maintain closing state of the combustion chamber. Further, thermal vacuum is generated in the combustion chamber due to pressure drop caused by decrease in temperature. Therefore, the piston can be moved toward its upper dead center because of the pressure difference between upper and lower spaces of the cylinder with respect to the piston. Such conventional power tool is described in for example, U.S. Pat. Nos. 4,522,162, 4,403,722, 4,483,473 and Re 32,452.
In the above-described conventional combustion type driving tool, generally, the cylinder, the combustion chamber frame, head cap those constituting the combustion chamber and a fan disposed therein are made from aluminum material, whereas the seal members such as O-rings are made from rubber and the housing is made from a plastic material. In this case, if the nail driving operation is repeatedly performed at a relatively short time interval, temperature of the combustion chamber frame and the cylinder are gradually elevated due to the heat absorption therein exceeding the cooling performance of the fan which generates cooling air in the combustion chamber frame and along the outer peripheral surface of the cylinder at the time of scavenging. This ultimately causes overheating. Therefore, cooling efficiency to the residual combustion gas in the combustion chamber is lowered, which affects generation of sufficient thermal vacuum. Consequently, return movement of the piston after nail driving is retarded, which lowers a sequential operation cycle thus degrading driving efficiency.
If nail driving operation is further continued in this overheating state, the housing and the handle those positioned adjacent to the combustion chamber frame and the cylinder are also heated. This heats the user's hand grasping the handle, to further degrade the operability. Finally, the excessive heat damages to the seal members which have been hermetically sealed the combustion chamber, because the seal members are made from thermally low heat resistant material such as rubber to lower the sealability, which leads to constant air introduction into the combustion chamber. This prevents the combustible gas from being ignited. Thus, no nail driving operation is performed in spite of manipulation to the trigger switch. Further, the housing may be thermally deformed and damaged because the housing is made from the plastic material. If the housing and/or seal members are damaged, the tool must be subjected to overhauling so as to replace the housing and the seal members by new housing and new seal members.
Further, the excessive temperature elevation lowers driving energy. If additional fan is supplemented for enhancement of cooling performance, additional installation space is required. Therefore, a compact tool cannot be provided.